1. Field of the Invention
The invention relates to a process for producing contact strips on substrates, especially on plates of mineral glass, which have an electrically conductive surface coating which is covered on the side facing away from the substrate with at least one surface coating of a dielectric material.
2. Discussion of Related Art
The products of such a process often serve, especially when the substrates consist of plates of mineral glass, as the windshields or back windows of motor vehicles. The electrically conductive surface coating, which in this case usually consists of a thin metal coating of high transparency in the visible part of the light spectrum, can be used as a heating resistance for the purpose of melting frost and evaporating moisture due to condensation. It is therefore necessary to provide the resistance coating with appropriate terminal contacts which will assure the most uniform possible distribution of the heat over the entire substrate surface and prevent local overheating in the immediate area of the terminal.
It is additionally desirable, especially in automobile glazing, for the surface coating to reflect the longer-wave-length portion of the sunlight spectrum--the so-called thermal radiation. This thermal radiation is especially undesirable in modern passenger cars with sloping and therefore large-area windshields and back windows, because they can heat the interior of the motor vehicle to intolerable temperatures.
Plates, including those of mineral glass, having such spectral transmission and reflection characteristics, are disclosed in EP-PS 35 906, DE-OS 33 07 661 and EP-OS 104 870. These publications correspond to respective U.S. Pat. Nos. 4,413,877 to Suzuki et al, 4,548,691 to Dietrich et al, and 4,462,883 to Hart. In the known plates, a silver layer having on one or both sides a diffusion barrier composed of a thin metal coating is sandwiched between two oxide layers of which one is a surface layer which protects the metal layers against chemical and/or mechanical attack. While the metal layers, especially the silver layer, have a sufficient electrical conductivity to serve as a heating resistance (at least as long as the plate is not heated to temperatures above 150.degree. C), the oxide surface layer is a definite insulator which prevents electrical contact with the metal layer or layers underneath it. It is extremely difficult to remove the relatively hard oxide surface coating to a defined thickness for the purpose of providing the metal layer beneath it with contact strips.
Therefore the invention is addressed to the problem of devising a process for the production of contact strips on substrates of the kind described above, which will result in a reliable and stable contact with the electrically conductive surface coating.
The solution of the stated problem is achieved according to the invention in the process outlined above by applying a noble metal suspension in a liquid to the surface coating (of a dielectric material) in the pattern of the contact strips, and exposing the substrate to a heat treatment at least 100.degree. C., until a lowered total resistance occurs through the conductive surface coating between the contact strips.
Noble metal suspensions of this kind are commercially obtainable, for example under the name "Leitsilber 200" of the firm Demetron in Hanau, Federal Republic of Germany.
It is known through the book by Dr. A.F. Bogenschuetz, "Oberflaechentechnik und Galvanotechnik in der Elektronik," Eugen G. Leuze Verlag, Saulgau/Wuerttemberg, 1971 edition page 298, to use paste-like suspensions of finely divided silver powder in a lacquer substance in order to make electrical contacts on insulating substances. It is also known through the same literature that the conductivity of such suspensions can be substantially improved by thermal aging. Leitsilber (conductive silver) is a silver colloidally dissolved in lacquer, which is applied and dried. An alternative method is to suspend silver powder in a casting resin (Araldit) in order to improve adhesive strength.
Such suspensions are also suitable for the purpose of the present invention. However, when a noble metal suspension of this kind was applied to a set of layers of the kind described above, it was surprisingly found that the noble metal, especially silver, diffuses through the dielectric layer and into the electrically conductive surface coating (of silver for example) on account of the heat treatment, and enters into an intimate bond or interlock with the said layers, which not only assures a stable passage of current, but also an excellent strength of adhesion of the contact strips in question.